Hi everyone, sorry if this is obvious or if it's right in front of my eyes and I can't see it.

I'm having a few problems understanding this, in this sentence, the hiragana character used for "wa" is "ha", because according to my source it's pronounced that way in that case.

neko dewa arimasen.

In hiragana however it was written as:

neko deha arimasen.

I'd very grateful if someone told me what's the difference on that, and what's used when japanese people talk between themselves and when they write (formal speech).

Also, about dewa arimasen and janai desu, I know the first is more formal than the second one, but when talking to people, which ones should I use in what situation? (Like with a new person I've met, or a friend, or a relative and so on..).

And what do these mean alone (or how can they be used with other words)? Like "dewa" and "janai", since they're different words I thought there would be more to it (even though I read and learned to use them together to create a negative sentence).

Thank you.. and please don't make it too complex (I'm a beginner student on my own, hopefully I won't give up so soon on japanese lol).

First, I'm just gonna say I think you're better off just accepting that's how it is for now, or you might turn yourself off to the language. I really think that sometimes, "suspension of belief" is better than asking "why" all the time.

HOWEVER.. I will provide what I can, and I hope I am not too confusing.... if you're SURE you want the "whys" to these questions, read on. Otherwise, just stop now, and come back later. :D

「は」 is a particle from ages ago. It used to be pronounced as 'ha'. But as language evolved, it became easier to say 'wa'. So now it is still spoken 'wa' but the written isn't changed.

'dewa' and 'arimasen' really dont mean anything by themselves... this gets kinda complicated.. I'll try to make it simple....

OK... the positive version is:
である (dearu).

You should know 'aru' as meaning "exists". It tells you that there IS *something*. Neko ga aru. "There is a cat."

"dearu" has the effect of describing a state of being. It can be used with nouns, but it has a slightly different meaning.

Neko ga aru. There is a cat.
Neko dearu. It is a cat. (literally: It's in the state of being a cat)

With an adjective:

Kowai dearu. It's scary!

Past tense of 'aru' is 'arimasen'.

Neko ga arimasen. "There is no cat."

When negating 'dearu', you must include a topic particle. So now we have the [very formal] "dewaarimasen"

In informal, quick speech, 'dewa' is shortened to 'ja' and arimasen shortened to 'nai'. So you have "janai"

Neko janai. "[It] isn't a cat." (literally: Not in the state of being a cat)
Kowai janai. "[It] isn't scary." (Not in the state of being scary)

You -can- use 'janai desu' but in reading mangas, watching animes, and watching japanese dramas, if they use ja nai, they don't bother with the desu. If it's informal enough for ja nai in the first place, tacking 'desu' to the end is probably unnecessary.

You can go mid-way with dewanai or jaarimasen (I don't hear this second one much in my limited experience).